Measurement of frequency (or wavelength) of an optical source is generally useful in a wide variety of situations, and is particularly important in applications where very precise control of laser output frequency is required, such as high precision spectroscopy. Accordingly, various methods for optical frequency monitoring have been developed to date. One such method is the use of one or more etalons to provide suitable signals for frequency monitoring. Etalons are attractive for frequency monitoring because they can provide high accuracy and precision without being unduly large or expensive.
One type of etalon frequency monitor relies on the use of two etalon signals in quadrature, because the use of two quadrature signals helps avoid reduced measurement sensitivity at local maxima or minima of the etalon signals. One example of this kind of approach is described in U.S. Pat. No. 6,859,284 and in U.S. Pat. No. 6,331,892, where detectors are disposed in an etalon interference pattern at a quadrature separation from each other. In U.S. Pat. No. 6,178,002, two beams take slightly different paths through an etalon, where the difference in path length is selected such that signals corresponding to the two beams are in quadrature.
The specific issue of how to derive a frequency reading from measured quadrature etalon signals has also been investigated. For example, in US 2007/0195328, “dead zones” of reduced sensitivity for each etalon signal are identified. If an etalon signal is in its dead zone, it is not included in the frequency computation. In this manner, the frequency computation is based on one or both of the etalon signals, and relatively useless etalon signals are systematically excluded from the computations, thereby improving performance.
However, the approach of defining dead zones in this manner can encounter significant difficulties in practice. In particular, switching from the use of one etalon signal to the use of two etalon signals at a dead zone boundary can lead to an undesirable situation where the frequency monitor error (i.e., the difference between monitor readout and actual frequency) is discontinuous. Although such discontinuities can be removed in calibration, they can reappear as a result of calibration drift. A discontinuous monitor error is particularly undesirable in situations where the monitor is part of a frequency control loop, since the loop may become unstable, or it may be impossible to access the target frequency at all.
Accordingly, it would be an advance in the art to derive frequency readings from a frequency monitor having two etalon signals in quadrature in a manner that provides high precision and accuracy and also inherently provides a continuous monitor error as a function of frequency.